depends. there are over 100.
For modern art, Mamba and Malba, and Bellas Artes.
I always take people to the Museo Xul Solar, which is a small beautiful museum dedicated to one artist.
I like the train museum behind Retiro Station, but I am a metal guy.
The Museo del Bicentenario, behind the Casa Rosada, is interesting and not overwhelming.
there are museums of humor, cars, cameras, clothes, printmaking, natural sciences, and opera sets and costumes.
Museo de Arte Decorativo is a beautiful palace, usually interesting shows, and a very nice little cafe in the gardens.